Service discovery via DNS in Kubernetes - Step-by-Step Execution

Process Flow - Service discovery via DNS

Pod starts

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Pod queries DNS for service name

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DNS server receives query

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DNS server looks up service IP

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DNS server returns service IP

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Pod connects to service IP

Pods use DNS to find services by name. The DNS server resolves the service name to its IP, enabling communication.

Execution Sample

Kubernetes

kubectl run busybox --image=busybox --rm -it --restart=Never -- sh -c "nslookup my-service && ping my-service"

A pod runs a busybox container, then queries DNS for 'my-service' and pings it to test connectivity.

Process Table

Step	Action	Input/Query	DNS Response	Pod Behavior
1	Pod starts	-	-	Pod is running and ready
2	Pod runs nslookup	Query: my-service.default.svc.cluster.local	Response: 10.96.0.10	Pod receives IP of service
3	Pod runs ping	Target: my-service	Uses IP 10.96.0.10	Pod sends ICMP packets to service IP
4	Ping succeeds	-	-	Pod confirms service is reachable
5	Pod stops querying	-	-	Service discovery complete

💡 Pod successfully resolves service name to IP and confirms connectivity

Status Tracker

Variable	Start	After nslookup	After ping	Final
service_name	my-service	my-service	my-service	my-service
service_ip	unknown	10.96.0.10	10.96.0.10	10.96.0.10
ping_status	not started	not started	success	success

Key Moments - 2 Insights

Why does the pod use the full DNS name 'my-service.default.svc.cluster.local' during lookup?

The pod appends the namespace and cluster domain to the service name to form the full DNS name, ensuring the DNS server can resolve it correctly. This is shown in execution_table step 2 where the query includes the full domain.

What happens if the DNS server does not respond with an IP?

The pod cannot resolve the service name, so it cannot connect. This would be seen as no IP in the DNS Response column in execution_table step 2, causing ping to fail in step 3.

Visual Quiz - 3 Questions

Look at the execution table, what IP does the pod receive for 'my-service' at step 2?

A 127.0.0.1

B 192.168.1.1

C 10.96.0.10

D No IP received

💡 Hint

Check the DNS Response column in execution_table row 2

At which step does the pod confirm that the service is reachable?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for 'Ping succeeds' in the Action column of execution_table

If the pod could not resolve the service IP, what would change in the variable_tracker?

A ping_status would be 'success'

B service_ip would remain 'unknown' after nslookup

C service_name would change

D service_ip would be '10.96.0.10'

💡 Hint

Check the service_ip row in variable_tracker after nslookup

Concept Snapshot

Service discovery in Kubernetes uses DNS.
Pods query DNS with full service name.
DNS returns service cluster IP.
Pods use IP to connect to service.
If DNS fails, connection fails.

Full Transcript

In Kubernetes, pods find services by asking the DNS server for the service's IP address. When a pod starts, it can run a DNS query like nslookup for the service name, which includes the namespace and cluster domain. The DNS server responds with the service's cluster IP. The pod then uses this IP to connect, for example by pinging it. If the DNS lookup fails, the pod cannot reach the service. This process allows pods to discover services dynamically by name.